Active illuminations are implemented in many camera designs to improve the camera measurement results. Camera is understood as an electro-optical device containing an image sensor with an array of pixels.
In many machine vision applications, active illuminations are used to guarantee for certain light intensity level on the camera system. By doing so. the performance of an active system becomes less prone to lighting changes in its surrounding. The stability and reliability is improved. In addition, by adding illumination power to the system, shorter exposures can be set and higher frame rates achieved.
Other sensors with an active illumination camera system use specific properties of the object of interest in the scene, e.g. an application might use reflectors in the scene that can easily be recognized and possibly tracked.
Other applications are using specific reflection properties of the object of interest such as the reflection properties of eyes. The typical back-reflection of the eyes captured by a system with active illumination can be used to detect and track eyes and to count the eye blinking to e.g. check a driver's drowsiness and build a fatigue measurement sensor.
In interferometers, an active system illuminates the object of interest and a reference target. Based on the interference of the reflections, depths can be measured and analyzed.
Other active systems modulate the illumination to get more information about its surroundings. Such illuminations are typically modulated in time (temporal modulation) or in space and can be applied to e.g. measure distances to map the environment in 3D. Temporal modulation is implemented in so-called time-of-flight (TOF) cameras (indirect and direct); spatial modulation is used in triangulation-based depth measurement system, also called structured light technology.
Time of flight image sensor pixels are dedicated pixels designs to guarantee an extremely fast transfer of the photo-generated charges to their storage nodes. Higher modulation frequencies result in better depth noise performance. Therefore, TOF demodulation pixels typically work in the range of a few tens of MHz up to a several hundreds of MHz. Further, TOF pixels often include background light suppression on pixel-level, in order to increase the dynamic range of the TOF imaging system. In most implementations, TOF pixels include two storage nodes to store and integrate two sampled signals per pixel.